docs: shuffle files to have the same logical hierarchy as on the generated doc

Also:
 - rename some files to remove redundant information from the filenames
 - rename many labels to match the filename and its place in the hierarchy
 - move lua_api under the scripting section

docs/rst/scripting/lua_api/lua_introduction.rst

@@ -0,0 +1,63 @@
+.. _lua_introduction:
+
+Introduction
+============
+
+`Lua <https://www.lua.org/>`_ is a powerful, efficient, lightweight,
+embeddable scripting language.
+
+WirePlumber uses `Lua version 5.4 <https://www.lua.org/versions.html>`_ to
+implement its engine. For older systems, Lua 5.3 is also supported.
+
+Scripts can be ran with the ``wpexec`` tool.
+
+Example scripts can be found in the `tests/examples` directory of the wireplumber source tree.
+
+Lua Reference
+-------------
+
+If you are not familiar with the Lua language and its API, please refer to
+the `Lua 5.4 Reference Manual <https://www.lua.org/manual/5.4/manual.html>`_
+
+Sandbox
+-------
+
+WirePlumber's scripting engine sandboxes the lua scripts to a safe environment.
+In this environment, the following rules apply:
+
+  - Scripts are isolated from one another; global variables in one script
+    are not visible from another, even though they are actually executed in
+    the same ``lua_State``
+
+  - Tables that hold API methods are not writable. While this may sound strange,
+    standard Lua allows you to change standard API, for instance
+    ``string.format = rogue_format`` is valid outside the sandbox.
+    WirePlumber does not allow that.
+
+  - The standard Lua API is limited to a subset of safe functions. For instance,
+    functions that interact with the file system (io.*) and the process's state
+    (ex.: os.exit) are **not** allowed.
+
+    Here is a full list of Lua functions (and API tables) that are exposed:
+
+    .. literalinclude:: ../../../../modules/module-lua-scripting/wplua/sandbox.lua
+      :language: lua
+      :lines: 27-30
+
+  - Object methods are not exposed in public tables. To call an object method
+    you must use the method call syntax of Lua, i.e. ``object:method(params)``
+
+    The following, for instance, is **not** valid:
+
+    .. code-block:: lua
+
+       -- this will cause an exception
+       local node = ...
+       Node.send_command(node, "Suspend")
+
+    The correct form is this:
+
+    .. code-block:: lua
+
+       local node = ...
+       node:send_command("Suspend")